Most owners of Mercedes don't know that their vehicle has a secondary battery.  This battery is responsible for providing back-up power to limited devices in the event that your main battery dies.  Such devices are primarily the transmission shift module.   Since the gear selector is electronic, it needs power to allow operation of shifting out of park into neutral.

To change this battery; it is located beneath the front passenger seat to the right of the main battery.  Unfortunately you also have to remove the main battery in order to gain access and space to remove the auxiliary battery.